it is called an \(X\)  window. Here, \(\psi _{AB}\)  can take an arbitrary value, which can be different from time to time as Alice and Bob like, so as to obtain a satisfactory key rate for the protocol [1]}. \(\lambda \)  is a positive value close to 0, and would be optimized to obtain the highest key rate. One-detector-heralded events in \(X\)  windows are called effective events.
